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+=============================
+Subsystem Trace Points: power
+=============================
+
+The power tracing system captures events related to power transitions
+within the kernel. Broadly speaking there are three major subheadings:
+
+  - Power state switch which reports events related to suspend (S-states),
+    cpuidle (C-states) and cpufreq (P-states)
+  - System clock related changes
+  - Power domains related changes and transitions
+
+This document describes what each of the tracepoints is and why they
+might be useful.
+
+Cf. include/trace/events/power.h for the events definitions.
+
+1. Power state switch events
+============================
+
+1.1 Trace API
+-----------------
+
+A 'cpu' event class gathers the CPU-related events: cpuidle and
+cpufreq.
+::
+
+  cpu_idle		"state=%lu cpu_id=%lu"
+  cpu_frequency		"state=%lu cpu_id=%lu"
+  cpu_frequency_limits	"min=%lu max=%lu cpu_id=%lu"
+
+A suspend event is used to indicate the system going in and out of the
+suspend mode:
+::
+
+  machine_suspend		"state=%lu"
+
+
+Note: the value of '-1' or '4294967295' for state means an exit from the current state,
+i.e. trace_cpu_idle(4, smp_processor_id()) means that the system
+enters the idle state 4, while trace_cpu_idle(PWR_EVENT_EXIT, smp_processor_id())
+means that the system exits the previous idle state.
+
+The event which has 'state=4294967295' in the trace is very important to the user
+space tools which are using it to detect the end of the current state, and so to
+correctly draw the states diagrams and to calculate accurate statistics etc.
+
+2. Clocks events
+================
+The clock events are used for clock enable/disable and for
+clock rate change.
+::
+
+  clock_enable		"%s state=%lu cpu_id=%lu"
+  clock_disable		"%s state=%lu cpu_id=%lu"
+  clock_set_rate		"%s state=%lu cpu_id=%lu"
+
+The first parameter gives the clock name (e.g. "gpio1_iclk").
+The second parameter is '1' for enable, '0' for disable, the target
+clock rate for set_rate.
+
+3. Power domains events
+=======================
+The power domain events are used for power domains transitions
+::
+
+  power_domain_target	"%s state=%lu cpu_id=%lu"
+
+The first parameter gives the power domain name (e.g. "mpu_pwrdm").
+The second parameter is the power domain target state.
+
+4. PM QoS events
+================
+The PM QoS events are used for QoS add/update/remove request and for
+target/flags update.
+::
+
+  pm_qos_add_request                 "pm_qos_class=%s value=%d"
+  pm_qos_update_request              "pm_qos_class=%s value=%d"
+  pm_qos_remove_request              "pm_qos_class=%s value=%d"
+  pm_qos_update_request_timeout      "pm_qos_class=%s value=%d, timeout_us=%ld"
+
+The first parameter gives the QoS class name (e.g. "CPU_DMA_LATENCY").
+The second parameter is value to be added/updated/removed.
+The third parameter is timeout value in usec.
+::
+
+  pm_qos_update_target               "action=%s prev_value=%d curr_value=%d"
+  pm_qos_update_flags                "action=%s prev_value=0x%x curr_value=0x%x"
+
+The first parameter gives the QoS action name (e.g. "ADD_REQ").
+The second parameter is the previous QoS value.
+The third parameter is the current QoS value to update.
+
+And, there are also events used for device PM QoS add/update/remove request.
+::
+
+  dev_pm_qos_add_request             "device=%s type=%s new_value=%d"
+  dev_pm_qos_update_request          "device=%s type=%s new_value=%d"
+  dev_pm_qos_remove_request          "device=%s type=%s new_value=%d"
+
+The first parameter gives the device name which tries to add/update/remove
+QoS requests.
+The second parameter gives the request type (e.g. "DEV_PM_QOS_RESUME_LATENCY").
+The third parameter is value to be added/updated/removed.